llm: register sdpa variant

[lanluo-nvidia]

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[peri044]

Minor comments. Please update the supported models section in README.md and docs/user_guide.

[peri044]

1. Could you quickly try running these variants as well ?
   a) google/gemma-3-4b-it
   b) google/gemma-3-270m-it
2. Please update the supported model list here as well: https://github.com/pytorch/TensorRT/blob/main/docsrc/tutorials/compile_hf_models.rst
3. Could you add a testcase for 1 Gemma-3 decoder layer with sliding window attention ? The test case could be located at https://github.com/pytorch/TensorRT/tree/main/tests/py/dynamo/models as test_llm_models.py